Pulse generators are known in the art and frequently output squarewave signals with a frequency proportional to the rate of rotation of an input shaft. Such pulse generators typically incorporate a sensor which produces a sinusoidal signal with a frequency proportional to the rate of the input shaft. The sinusoidal signal is conditioned and converted to a train of output squarewave signals for transmission to counters and controllers and the like. An ideal squarewave may, for example, have a 50% duty cycle. The duty cycle is typically defined as the percentage of time the output signal is high or ON relative to the time for one complete cycle of the sinusoidal signal. The circuitry employed frequently superimposes the sine wave on a DC offset. The signal conditioning circuitry that converts the sensor sine wave to a squarewave signal must be adjusted to compensate for this offset. This can be accomplished with selected resistors or potentiometers. Once adjusted, however, if the sensor DC offset changes from aging, temperature or any other environmental factors, there is a detrimental impact on the output squarewave duty cycle. The deterioration of the output duty cycle will continue until it reaches a failure condition for a particular application and the rotary pulse generator would need to be replaced.
One method known in the prior art for compensating for sensor DC offset changes is to provide temperature compensation. This requires that parts with opposing temperature coefficients be matched together for compensation. This technique has limitations in that the matches are never perfect. The control in this technique is open loop and the duty cycle is only indirectly corrected for as it does not take into account aging and other environmental factors.
Another method of compensation known in the art may be referred to as capacitor coupling. In this technique, the sinusoidal output of the pulse generator sensor is passed through a series capacitor that blocks the DC portion of the signal. The limitation of this technique is that on start up the capacitor must be charged and several cycles must pass before an output signal is generated.
It has been known in the prior art to try to adjust the duty cycle of the pulse generator based on looking at the input conditions only. It is believed that no attempts have been made in the prior art to try to adjust the duty cycle based on looking at the output conditions of a comparator to adjust the threshold thereof.
The M. Suquet U.S. Pat. No. 4,902,970 discloses a rotary pulse generator which provides a quasi-sinusoidal signal with the frequency being related to the speed of shaft rotation. The sinusoidal signal is supplied to a comparator which is employed for purposes of providing a train of squarewave pulses. This patent does not appear to recognize the need to control the duty cycle of the squarewave signal because of variations in the DC offset with time and temperature as recognized by the invention herein. Moreover, there is no suggestion in this patent that the duty cycle be controlled by varying the threshold signal.